1. Field of the Invention
This invention relates to centrifugal separation apparatus.
This invention has particular but not exclusive application to a continuous centrifuge for separating sugar crystals from a sugar solution, or massecuites and it will therefore be convenient to hereinafter refer to this example application. However It is to be clearly understood that this invention could be used in other applications for separating other solid/liquid mixtures.
2. Brief Description of the Art
In the processing of sugar crystals, a filtration centrifugation separation process, eg a centrifuge, is frequently employed to separate sugar crystals from a sugar solution. This is especially so for highly pure crystal sugar for consumption or refining where crystal quality is important.
In most known industrial applications, the separation of sugar crystals from massecuite and washings is carded out in batch processes using batch centrifuges. In a batch centrifuge, a predetermined charge of massecuite is delivered to a centrifuge basket. The basket is then accelerated to a predetermined angular velocity and held for a predetermined spin time to achieve a desired level of syrup removal before wash water and/or an alternative liquid is sprayed onto the exposed crystal layer for a predetermined "wash time". The basket is then hold at the aforementioned, or an alternative, angular velocity before decelerating the basket and discharging the washed crystals therefrom.
Some advantages of the batch centrifuge are that crystal size and shape are preserved since the crystals do not undergo any high velocity impacts and that by appropriate selection of spin times and wash times, the required level of residual impurities can be controlled. However these machines require time to be charged with feedstock and emptied of product and also require time to accelerate and decelerate thus reducing their effective capacity, Thus the batch operation imposes a considerable down time thereby requiring a relatively larger number of filtration centrifugation units to satisfy the throughput of a sugar crystallization plant.
Maintenance requirements are significant due to the cyclic operation of the equipment and power consumption is increased due to the inefficiency of the cyclic operation.
Attempts have been made to develop continuous centrifuges suitable for producing high quality sugar crystals. However these attempts have not produced very satisfactory results. A major problem with known continuous centrifuges is crystal breakage caused by high speed impacts between crystals leaving the conical basket and the walls of a sugar chamber and also crystal impacts with other crystals. In addition, wet lumps of sugar may agglomerate and these naturally adversely affect sugar crystal quality.
As a result continuous centrifuges have not been widely used up to now for high purity product sugar separation. However in view of the obvious shortcomings of batch centrifuges as articulated above, It would clearly be advantageous if a continuous centrifuge could be devised which preserved a good sugar crystal quality, eg suitable for consumption and use in refining.